Radiotherapy using high-energy X-ray radiation, electrons, protons or heavy ions is regularly used in order to treat tumors. With this kind of radiotherapy a target volume inside a patient is irradiated, with the tumor being at least partially located in the target volume. The target volume should be defined as precisely as possible, so the desired effect of the radiotherapy is primarily achieved in the target volume.
It is customary to record tomographic planning data of the patient in advance of radiotherapy for the planning thereof and for defining the target volume. Raw data is typically recorded using a computer tomograph, with the planning data being reconstructed from the raw data. The planning data comprises a plurality of planning images. It is possible to derive from this planning data how the anatomy of the patient and the position of the tumor change due to the breathing of the patient. Since, as a rule, radiotherapy occurs continuously it is important to take into account the change in the anatomy due to the breathing of the patient when planning radiotherapy and defining the target volume.
In order to associate the change in the anatomy with the course of the respiratory cycle of the patient, the breathing of the patient is likewise recorded with the aid of a breathing surrogate during the tomographic scanning. A breathing surrogate is a scanning system which is configured to record the respiratory cycle of the patient. For example, a breathing surrogate can be implemented by a chest strap or by a transducer attached to the chest of the patient. A breathing surrogate can also be implemented by a device for measuring the temperature of the breath of the patient.
Various approaches exist for reconstructing the planning data and for correlating this planning data with the respiratory cycles. The correlation can occur, in particular, by way of a phase-based method and by way of an amplitude-based method. In a phase-based method the raw data is selected for the reconstruction such that the respiratory cycles of the patient are scanned in a temporally equidistant manner. The planning data is therefore reconstructed such that adjacent planning images each have the same interval. In an amplitude-based method the raw data is selected for the reconstruction such that the amplitudes of the respiratory cycles of the patient are scanned equidistantly. Adjacent planning images can then have different intervals from each other.
Phase-based methods are more susceptible to irregular breathing of the patient and therefore have a greater number of and more pronounced artifacts than amplitude-based methods. Phase-based methods are primarily susceptible to variable plateaus during exhalation and to variable maximum amplitudes during inhalation and to variable breathing rates. Temporally equidistant planning images are desirable for certain types of planning of radiotherapy, however. It is therefore an object of the present invention to compensate for the drawback of known phase-based methods compared to amplitude-based methods. In particular, a method for the correlation of planning data with respiratory cycles of the patient shall be disclosed which is less susceptible to irregular breathing of the patient.